HEARING 



1065 



nerves and of giving rise to a sensation of intermittent pressure or thrill. 

 This is readily perceived when the finger is immersed in a vessel of 

 water into which dips a tube connected with a source of sound, or when 

 a vibrating bell or tuning-fork is touched. So far as we know, what 

 takes place in the ear is essentially similar that is to say, a mechanical 

 stimulation of the ends of the auditory nerve, but a stimulation which 

 acts through, and is graduated and controlled by, a special intermediate 

 mechanism. 



As the visual apparatus consists of a sensitive surface, the retina, 

 which contains the end-organs of the optic nerve and of dioptric 

 arrangements which receive and focus the rays of light, the auditory 

 apparatus consists of the sensitive end-organs of the cochlear divi- 

 sion of the eighth nerve and of a mechanism which receives the 

 sound-waves and communicates them to these. 



Physiological Anatomy of the Ear. At the bottom of the external 

 auditory meatus lies the membrana tympani, a nearly circular mem- 

 brane set like a drum-skin in a 

 ring of bone, and separating the 

 meatus from the tympanum or 

 middle ear. Its external surface 

 looks obliquely downwards, and 

 at the same time somewhat for- 

 wards, so that if prolonged the 

 membranes of the two ears would 

 cut each other in front of, and also 

 below, the horizontal line passing 

 through the centre of each (Figs. 



453.454)-" , 



The tympanum contains a 

 chain of little bones stretching 

 right across it from outer to inner 

 wall. Of these the malleus, or 

 hammer, is the most external. 

 Its manubrium, or handle, is in- 

 serted into the membrana tym- 

 pani, which is not stretched taut 

 within its bony ring, but bulges 

 in wards .at the centre, where the 

 handle of the malleus isattached. 

 The stapes, or stirrup, is the most 

 internal of the chain of ossicles, 

 and is inserted by its foot-plate 

 into a small oval opening the 



foramen ovale on the inner wall of the tympanic cavity. A mem- 

 branous ring the orbicular membrane surrounds the foot of the 

 stapes, helping to fill up the foramen and attaching the bone to its 

 edges. The inner surface of the foot of the stapes is in contact with 

 the perilymph of the internal ear. The incus, or anvil, forms a link 

 between the malleus and the stapes. The auditory ossicles, as well as 

 the whole cavity of the tympanum, are covered by pavement epithelium. 



The tympanum is not an absolutely closed chamber; it has one 

 channel of communication with the external air the Eustachian tube 



which opens into the pharynx. By the action of the cilia lining this 



tube the scanty secretion of the middle ear is moved towards its 

 pharyngeal opening, which, usually closed, is opened when a swallowing 



Fig. 453.^The Ear. m, external meatus; 

 /, head of malleus; o, short process of 

 malleus; g, handle of malleus; h, incus; 

 i, foot of stapes in oval foramen; e, tym- 

 panic membrane. 



